Side project - MV864A meter restoration - converting manual calibration/test instructions to the version I own

TEST POINTS HAVE CHANGED SIGNIFICANTLY BETWEEN VERSIONS

When Millivac re-engineered the meter from the germanium transistor technology design that I own to make use of more modern silicon semiconductors and more precise resistor values, they renumbered everything. There are fewer coupling transformers and more transistors in the new design that is reflected in the manual I purchased, in addition to every part having a different number. 

The manual's power supply test points 4TP1, 4TP2 and 4TP3 became TP62, ground and TP63 on my schematic. The manual's test points on the main board are:

• 3TP1
• 3TP2
• 3TP3
• 3TP4
• 3TP5
• 3TP6
• 3TP7
• 3TP8
• 3TP9
• 3TP10
• 3TP12
• 3TP13

My meter has test points:

• TP21
• TP22
• TP23
• TP24
• TP25
• TP26
• TP27
• TP28

The version in the manual has seven jumper wires soldered on the board, which can be disconnected to isolate the different sections for debugging.  On the schematic in the manual, there are wire 3W1, 3W2, 3W3, 3W4, 3W5, 3W6, and 3W7. On my board, there is a wire soldered across TP22 and TP23, plus points TP24 and TP28 have posts upon which wire could be soldered.

This is important as the testing recommendations jumper 3W3 is opened to check the modulator and preamplifiers. To check the main amplifier, jumper 3W4 is added between 3TP3 and 3TP13 while jumper 3W3 is relocated to bridge 3TP9 and 3TP10. 

On my meter, the equivalent of 3W3 is the wire soldered between TP22 and TP23, thus removing it would let me check the modulator and preamplifier sections. I don't see anything corresponding to how the jumper 3W4 would be used on my meter to test the main amplifier. 

WAVEFORM OBSERVATION POINTS

With the wire removed between TP22 and TP23, I should be able to adjust the zero control pot R257A to get a minimum amplitude waveform at TP22 with no signal applied to the meter. Inputting a 100 microvolt signal to the meter should produce a 400mV peak to peak 94Hz almost sine wave at TP22.

It is less clear how to monitor the output of the reference amplifier since the circuit is so different but my guess is that I should monitor at TP28 where I should see a 46V peak to peak 94Hz signal with the 100 microvolt input to the meter. The method of adding jumper 3W4 and moving 3W3 is used to adjust the gain potentiometer, but I don't see at this time how I could achieve a similar test that would produce meter needle sitting between 45 and 55% of full scale. 

Checking the driver circuit should involve looking at TP24 where I would see a very low signal at 3V peak to peak with a zero input to the meter and a 94Hz rectified signal at 5V peak to peak when there is a 100 microvolt input to the meter. However the circuit is different enough that I won't immediately consider that I have a fault if the waveform doesn't match the manual. 

POWER RAIL RELABELING

The power supply in the manual has test points 4TP1, 4TP2 and 4TP3, while the boards on my meter have points TP62, TP63 and Ground. The manual labels those points as -6V, +8.5V and ground, whereas my test points register -14.5V and -6V against ground. The testing section of the manual tells me to measure between 4TP2 and 4TP1 adjusting a pot to set the voltage to -14.5 but on my meter I see -14.5V between TP63 and ground. 

The version in the manual seems to have reversed where the ground sits, in other words swapping the meaning of ground between 4TP2 and 4TP3. That makes it even harder to reconcile the schematics between the two, since the manual would show transistors with connections to +8.5V while on mine the same transistor would be connected to ground. 

CALIBRATION REMAINS THE SAME FORTUNATELY

The process of calibration involves applying reference voltages, currents and resistances while adjusting small pots on the rear of the meter to get the meter reading to match. There are multiple ranges for the meter each of which has its own adjustment pot for voltage ranges. Current ranges have multiple pots but several ranges are combined into one pot. 

Voltage adjustments are made at 100 microvolt, 300 microvolt, 1 millivolt, 3 mV, 10mV, 30mV, 100mV, 300mV, 1V, 3V 10V, 30V, 100V, 300V, and 1KV inputs. Current adjustments are made at .1 microamp, .3 microamp, 1 microamp, 3 microamp, 10 milliamp, 30mA, 100mA, 300mA, 1A and one adjustment for all ranges between 10 microamp and 3 mA. Resistance checks are made by setting the full scale ohms pot to infinity before each range measurement, but for the two lowest resistances one must first short the inputs and adjust the zero meter pot then set the full scale infinity pot before the measurement. 

A final linearity check is done on the 1V range while adjusting the input to the meter to 100mV, 200mV, 300mV, 400mV, 500mV, 600mV, 700mV, 800mV, 900mV and 1V while validating the needle position on the meter.